Primary Loop: Flash or Gravity? Both?

When the grinding circuit has two circulating loads, the authors advocate tapping the primary loop for gold recovery, either at the ball mill feed or cyclone underflow, rather than the secondary load. In all McGill surveys of plants that included primary and secondary classification, the primary cyclone underflow had a better potential for gravity recovery than the secondary cyclone underflow .

Coarse gold recovery flotation

For sulphidic base metal ores, flash flotation should be tested very seriously, because of the relatively low cost of installing a flotation unit compared to the potential benefits of increased NSR and lower reagent costs. For brownfield applications, the circulating load of gold can be an indicator of potential flash flotation performance (the higher the load the higher the potential gold recovery). There are indications that the benefits of flash flotation depend on the amount of GRG: for example, flash flotation at Macreas was not thought to be beneficial because of the low GRG content (most gold is present as gold carriers.

For gold ores that do not contain potentially cyanicidal base metal species, the use of flash flotation is not as common, and only makes sense if sulphide concentration is advisable. This is normally used in four cases:

  • Pre-concentration of the cyanidation feed (flotation tailing to be discarded)
  • Pre-concentration for subsequent oxidation (roasting, autoclaving, bioleaching) for refractory ores (flotation tailing to be cyanided)
  • Pre-concentration for direct intensive cyanidation
  • Recovery of gold-bearers and incompletely leached gold particles from a cyanidation tailing for finer grinding and intensive cyanidation.

Of the four options, the third option is relatively less common, and can be used in the case of high preg-robbing ores, in which case gravity rather than flotation should be used. When the third option is used for concentrating sulphides for finer grinding before cyanidation, flotation is an attractive option. For the fourth option, sulphides-carrier recovery by gravity is to be preferred over flotation -e.g. New Celebration.

With the second option, the main benefit of flash flotation is an increased recovery of the gold carrier (sulfide) by limiting over-grinding. This translates into a higher gold recovery. Without flash flotation over-ground gold bearing sulfides in the cyclone overflow are not recovered as efficiently in downstream flotation. Even when the flotation tailing is cyanided, the refractory nature of the gold present normally yields poor cyanide recoveries, typically much lower than what is achieved with the oxidized stream. Flash flotation may also assist in the production of a throwaway tailing from a downstream conventional flotation circuit, but this is unlikely in most cases.

Notwithstanding the number of cases that are discussed in the above paragraphs, the use of flotation in the absence of economically recoverable base metals is the exception rather than the norm. Political pressure to ban or reduce the use of cyanide may modify industrial practice in some countries or states and make flotation more attractive, in which case flash flotation would see more applications. Of the alternate leachants, of particular interest is the thiosulphate route, which may be easier to apply and thus more attractive on higher-grade feeds -i.e. gravity or flash flotation concentrates.

The use of both gravity recovery and flotation from the primary circulating load is restricted to very special cases where the attractiveness of recovering a smeltable precious metal concentrate (either to facilitate smelter settlements or maximize NSR) and maximizing metallurgical recovery coincide. It should understood that the two recovery methods will tap from the same circulating load, gravity recovery being the dominant recovery method above 212 mm and flotation below. Under such circumstances, gravity recovery should target mostly coarse gold and the use of a unit that can treat the full cyclone underflow without excessive dilution, even at the expense of fine gold recovery, is particularly appropriate.

At Macreas, the original flotation circuit incorporated column cells. These were unable to cope with the froth load imposed in floating both sulfide and carbonaceous material and consequently overall sulfide, and therefore gold, recovery decreased. Flash flotation of the carbonaceous components was implemented, which made it possible not only to improve downstream sulphides recovery, but also minimize the surface area of the carbonaceous material and hence its preg-robbing capabilities.

Table 1 summarizes the process options in the primary grinding loop.

Option Rationale Mills
Gravity alone Coarse gold recovery ahead of cyanidation GRG recovery ahead of base metal flotation to increase NSR and overall gold recovery Most gold mills Camchib, Osborne, Bulyanhulu
Flash flotation alone Increase base and precious metal recovery ahead of conventional flotation Louvicourt, Cadia, Lucien Beliveau
Gravity and flash flotation Base and precious metal recovery in the presence of coarse GRG MSV, Chimo, Eskay Ck., Ridgeway (planned)

 

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